New Year's Eve - once in a blue moon

There will be a blue moon on Thursday as New Year's Eve revellers welcome in 2010 - the first time since 1990 that a blue moon has coincided with New Year's Eve.

A blue moon - the second full moon in a calendar month - occurs only every 2½ years on average. This month, full moons occur on December 2 and December 31.

An astronomer, David Reneke, from Australasian Science magazine, said it was rare for the event to land on New Year's Eve, and it would not happen again until 2028.

''While everyone's celebrating they should also take a moment and look up into the night sky,'' Mr Reneke said.

But the moon will not turn blue - if anything, Mr Reneke said it could turn red when viewed from cities because of the filter effect of smoke from fireworks.

''It's not impossible that the fireworks will change the colour of the moon,'' he said.

He said the best way to view the blue moon was to get away from the city lights.

The phrase ''blue moon'' has become a metaphor for a rare event. The earliest English record of the expression dates back to a 1528 pamphlet which, in criticising the English clergy, read: ''If they say the moon is blue, we must believe that it is true.''

An alternative interpretation of the phrase suggests that it originates from the other old English meaning of ''belewe'' - which can mean the colour or a ''betrayer''.

Clergy identifies the Lent moon when calculating the dates for Easter. It is also thought that historically when the moon's timing was too early, they named the earlier moon as a ''betrayer moon'' or belewe moon.

However, Mr Reneke said the eruption of Krakatoa in 1883 spilled so much dust into the atmosphere that for almost two years afterwards the moon took on a bluish hue.

- © Fairfax NZ News